HIV is extremely adept in evading humoral immunity. Although neutralizing antibodies (NAbs) are produced to the viral envelope glycoprotein (Env), they are characteristically directed to hypervariable loops on gp120 (V1/V2 and V3), which can tolerate extensive genetic variation. Thus, these NAbs are generally "type specific" and not broadly neutralizing. In addition, the variable loops protect highly conserved functional domains on the gp120 core, i.e. binding sites for CD4 and chemokine receptors (CCR5 and CXCR4). Using CD4-independent Envs, we have derived replication competent variants of HIV-2 that lack V1/V2 and V3 hypervariable loops. Although viruses without V1/V2 have been described for HIV-1 and SIV, no virus to date has tolerated deletions in V3; V3 has been considered indispensable for Env function and coreceptor binding. Given the role of V1/V2 and V3 in protecting core domains from humoral immune responses, we hypothesize that Envs from these "minimized," loop-deleted viruses may have the potential to elicit novel antibodies to conserved and functional epitopes on gp120. Moreover, we hypothesize that Envs from CD4-independent HIV isolates represent a transition state in the Env-fusion cascade with exposed receptor binding regions and decreased conformational flexibility, properties that may be desirable in generating neutralizing antibodies. Thus, CD4-independent Envs with strategic removal of structures prohibitory to effective neutralizing antibody elicitation without altering Env function may allow derivation of a better immunogen that is able to elicit broadly neutralizing antibodies and "immune refocus" these antibodies towards key functional regions on Env. Our work has been focused on extending our findings to HIV-1 Envs such that they may be applied to HIV-1 vaccine development. Instead of taking a direct approach of generating HIV-1 Envs with hypervariable loop deletions, we explore here the feasibility of introducing HIV-1 converting mutations at receptor binding regions and/or epitopes for key anti-HIV-1 NAbs into our fully-functional hypervariable loop deleted HIV-2 Envs to generate functional, loop deleted "HIV-1 like" Envs for future HIV-1 vaccine studies. Two Specific Aims are proposed: #1) engineer HIV-1 converting mutations into wildtype HIV-2/vcp Env and functional, hypervariable loop deleted derivatives and assess for retention of functionality and #2) determine the reactivity of key anti-HIV-1 neutralizing MAbs (b12, CD4i MAbs, 2F5, and 4E10) to wildtype HIV-2/vcp Env and hypervariable loop derivatives with HIV-1 converting mutations.